Retardation films are used in STN (Super Twisted Nematic) systems of liquid crystal display devices and the like, and they are employed to solve such problems as color compensation and to achieve viewing angle widening. The materials generally used in retardation films for color compensation are polycarbonates, polyvinyl alcohol, polysulfone, polyethersulfone, amorphous polyolefins and the like, while the materials used in retardation films for viewing angle widening are those mentioned above, as well as polymer liquid crystals, discotic liquid crystals, and the like.
A quarter-wave plate, which is one type of retardation film, can convert circularly polarized light to linearly polarized light, or linearly polarized light to circularly polarized light. This has been utilized in liquid crystal display devices and, particularly, in reflective liquid crystal display devices having a single polarizing plate where the rear electrode, as viewed by an observer, is the reflecting electrode, in anti-reflection films comprising a combination of a polarizing plate and a quarter-wave plate, or in combination with reflective polarizing plates composed of cholesteric liquid crystals or the like that reflect only circularly polarized light only in either the clockwise direction or counter-clockwise direction.
The retardation films used in the aforementioned single polarizing plate-type reflective liquid crystal display devices and reflective polarizing plates must have a function of converting linearly polarized light to circularly polarized light and circularly polarized light to linearly polarized light, in the visible light region with a measuring wavelength of 400-700 nm, and preferably 400-780 nm. When this is accomplished with a single retardation film, the retardation film ideally has a retardation of λ/4 (nm) at a measuring wavelength λ of 400-700 nm, and preferably 400-780 nm.
Although the aforementioned color compensating retardation film materials are commonly used as quarter-wave plates, these materials exhibit birefringent wavelength dispersion. The birefringence of most polymer films becomes larger as the measuring wavelength becomes shorter, and becomes smaller at longer wavelengths. Consequently, with a single polymer film it has been difficult to achieve a smaller birefringence at shorter measuring wavelengths at a measuring wavelength λ of 400-700 nm, such as with the aforementioned ideal quarter-wave plate.
In order to achieve a smaller birefringence with shorter measuring wavelengths as with an ideal quarter-wave plate, Japanese Unexamined Patent Publication HEI No. 10-68816 has disclosed a technique of using a quarter-wave plate and a half-wave plate attached together at an appropriate angle, and Japanese Unexamined Patent Publication HEI No. 2-285304 has disclosed a technique whereby two retardation films with different Abbe numbers are laminated.
Current techniques require the use of two films in order to achieve a film with a smaller retardation with shorter measuring wavelengths as with ideal quarter-wave plates, and this has presented problems such as additional steps for film attachment and increased costs as well as greater expense for the optical design. In Japanese Unexamined Patent Publication HEI No. 3-29921 there is disclosed a retardation film obtained by uniaxially stretching a film composed of a mixture or copolymer of at least two different organic polymers, wherein the first organic polymer of the two different organic polymers has a positive photoelastic constant and the second organic polymer has a negative photoelastic constant, so that the retardation film has a larger birefringence at shorter measuring wavelengths; however, no reference is made to a method of reducing the birefringence at shorter measuring wavelengths. The present invention solves this problem by allowing realization of a retardation film with a smaller retardation, at shorter measuring wavelengths, using a single film.